In the art, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis. Such elastomers, when compounded to form articles such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance and will have less heat build-up when mechanical stresses are applied.
The main source of hysteretic power loss has been established to be due to the section of the polymer chain from the last cross link of the vulcanized to the end of the polymer chain. This free end cannot be involved in an efficient elastically recoverable process, and as a result, any energy transmitted to this section of the cured sample is lost as heat. It is known in the art that this type of mechanism can be reduced by preparing higher molecular weight polymers which will have fewer end groups. However, this procedure is not useful because processability of the rubber with compounding ingredients and during shaping operations decreases rapidly with increasing molecular weight.
Another method of reducing hysteresis has been to react a lithium endcapped elastomer with a tin chloride compound to give a polymer terminated with a tin compound. Tin has an affinity for carbon-black, which affinity reduces hysteresis by removing the effect of a free end. The present invention makes use of endcapped polymers which provide for even further reduction of hysteresis during the curing of the elastomer.
Other additives have been used to react with the carbon-lithium bond to reduce hysteresis characteristics. These have included heavy metal compounds and other organic compounds containing polar groups such as amines, amides, esters, ketones and various combinations thereof.
The present invention provides novel elastomers endcapped with a sulfoxide containing compound. The sulfur containing group of the endcapped elastomer can react with the carbon black or with another polymer backbone during cure of the elastomer, effectively reducing the number of end groups available to cause hysteresis.